noImplicitAny

There are some things that cannot be inferred or inferring them might result in errors that might be unexpected. A fine example is function arguments. If you don't annotate them its unclear what should and shouldn't be valid e.g.

So if you don't annotate some function argument TypeScript assumes any and moves on. This essentially turns off type checking for such cases which is what a JavaScript dev would expect but can catch people that want high safety off guard. Hence there is an option noImplicitAny that when switched on will flag the cases where the type cannot be inferred e.g.